The present invention relates generally to power and control systems and more specifically to power and control systems used to control the temperature of a workpiece such as a semiconductor wafer and/or to control the temperature of the workpiece chuck on which the workpiece is held.
In the semiconductor integrated circuit industry, the cost of individual integrated circuit chip die is continuing to decrease in comparison to IC package costs. Consequently, it is becoming more important to perform many IC process steps while the die are still in the wafer, rather than after the relatively expensive packaging steps have been performed.
Typically, in IC processing, semiconductor wafers are subjected to a series of test and evaluation steps. For each step, the wafer is held in a stationary position at a process station where the process is performed. For example, circuit probe testing is increasingly performed over a wide temperature range to temperature screen the ICs before assembly into a package. The wafer is typically held stationary relative to a vacuum support surface of a prober machine which electrically tests the circuits on the wafer. The prober includes a group of electrical probes which, in conjunction with a tester, apply predetermined electrical excitations to various predetermined portions of the circuits on the wafer and sense the circuits""responses to the excitations.
In a typical prober system, the wafer is mounted on the top surface of a wafer chuck, which is held at its bottom surface to a support structure of the prober. A vacuum system is typically connected to the chuck. A series of channels or void regions in communication with the top surface of the chuck conduct the vacuum to the wafer to hold it in place on the top surface of the chuck. The prober support structure for the chuck is then used to locate the wafer under the probes as required to perform the electrical testing on the wafer circuits.
The chuck can also include a temperature control system which raises and lowers the temperature of the chuck surface and the wafer as required to perform the desired temperature screening of the wafer. It is important to the accuracy of such testing that the temperature of the wafer and, therefore, the temperature of the chuck surface, be controlled as accurately and precisely as possible.
Various approaches to controlling the wafer temperature have been employed. In one prior system, the chuck includes a circulation system through which a cooling fluid is circulated. The cooling fluid is maintained at a constant cold temperature and is circulated through the chuck. Temperature control is realized by activating a heater which is also located in the chuck. The heater is cycled on and off as required to heat the chuck and the workpiece to the required temperature.
This approach has certain drawbacks. A large time lag occurs when heating the chuck because the circulation fluid is always maintained at a low temperature and is always circulating through the chuck. As a result, a large amount of time can be required to heat the chuck and workpiece to a high temperature. Also, the system can be inefficient since much of the energy provided to the heater is wasted in the presence of the circulating cold fluid. Additionally, energy used to cool the fluid is wasted when the chuck and workpiece are being heated.
In another prior system, both a temperature-controlled fluid and a chuck heater are used to control the workpiece temperature. In this system, the fluid is used to bring the workpiece to within a certain tolerance of the desired set point temperature. The heater is then cycled as required to trim the temperature to the set point. This system also suffers from long time lags and poor efficiency.
In still another prior art system, temperature control is implemented using only passive heat transfer to and from a fluid circulating through the chuck. In this system, the chuck is provided with a series of internal channels through which the temperature-controlled fluid is circulated. The chuck temperature is controlled by controlling the temperature of the fluid. This system also suffers from long time lags and relatively low efficiency.
In some applications, such as where a wafer is being tested on a circuit prober, it is important to reduce the electrical noise introduced into the system, since such noise can adversely affect the measurements being made by the prober. Introduction of noise into the measurement is a common problem where temperature testing of the wafer is being performed on the prober. Power and control signals applied to elements such as resistive chuck heaters are typically in close proximity to the wafer and, therefore, can be substantial sources of noise.
Therefore, it is important that power supplies that provide power to heating elements be as noise-free as possible. As a result, prior thermal chuck power systems include linear power supplies to provide power to heating elements. However, linear supplies tend to be very inefficient. In fact, their power dissipation is highly dependent on input voltage. Therefore, under conditions in which the input line power can vary, substantial inefficiency can result. Also, because the standard European line power voltage level is higher than that used in the U.S., the power dissipation of a linear supply would be higher in Europe than it would be in the U.S., thus requiring different supply and system designs or tolerance of substantial variation in power dissipation. In addition, linear supplies are not capable of power factor correction. Under new European standards soon to be implemented, high-power supplies must be power factor corrected. Linear supplies may not meet these new standards under certain conditions. Therefore, it would be desirable to have a workpiece chuck that is powered by noise-free power signals but does not rely on linear power supplies for power.
In one aspect, the present invention is directed to a temperature control system and method for controlling temperature of a workpiece such as a semiconductor wafer mounted on a workpiece chuck which provide increased temperature control accuracy and improved efficiency. The workpiece chuck has an upper portion on which the workpiece can be mounted. The upper portion is mounted over an intermediate layer such as a base which is located between the upper portion of the chuck and the host apparatus on which the chuck is being used, such as a wafer prober machine. The temperature control system includes means for controlling a temperature of the upper portion of the chuck to control the temperature of the workpiece and means for controlling a temperature of the base to affect heat flow between the chuck and the host apparatus. In one embodiment, the temperature of the base is controlled by a fluid circulated through the base. A portion of the fluid can also be circulated through the upper portion of the chuck to control the temperature of the upper portion of the chuck. In one embodiment, heat flow between the chuck and the host machine is reduced.
In one embodiment, the temperature control system of the invention includes a pump for circulating the fluid through the workpiece chuck. The system also includes means for transferring a portion of the fluid to the upper portion of the chuck to control the temperature of the workpiece and means for transferring another portion of the fluid to the base intermediate layer such that heat flow between the chuck and the host machine can be affected.
In one embodiment, the temperature control system includes a system for controlling the temperature of the fluid. This fluid temperature control system can include a refrigeration system through which the fluid flows to cool the fluid before it is pumped to the chuck. The fluid control system can also include one or more heaters for heating the fluid as required. The fluid control system can also include one or more valves or switches for controlling various fluid routing functions. For example, when it is required to heat the fluid, one or more valves may be controlled to cause the fluid to bypass the refrigeration system and/or to flow through one or more of the fluid heaters. This can result in substantial power savings by reducing unnecessary loads on the refrigeration system. In other circumstances, it may be desirable to cool the fluid. In this case, one or more valves can be controlled to route the fluid through the refrigeration system.
The valves can also be used to control whether the fluid is routed to the upper portion of the chuck to control the workpiece temperature or is routed to the base to set up the heat flow barrier between the chuck and the host machine. In one embodiment, two individually controllable flow paths are established. Under certain conditions, the valves are controlled to route a portion of the fluid to the upper portion of the chuck to control the workpiece temperature. At the same time, a second portion of the fluid can be routed to the base to maintain the base at or near ambient temperature and thereby prevent heat flow to and from the host machine. Under other conditions, the valves are controlled to route all of the fluid to the base.
In one embodiment, fluid heaters are provided in both fluid paths. The fluid heaters are individually controllable such that the temperatures of both portions of the fluid can be individually controlled.
The valves can also be used to control the amount of fluid circulated through the system. The pump can be bypassed by a controllable valve which, when activated, recirculates a predetermined portion of the fluid at the outlet of the pump back to its inlet to reduce the amount of fluid that reaches the chuck. The amount of circulating fluid can be thus reduced when the chuck is being heated, for example.
The temperature control system of the invention can also include one or more heaters in the chuck. The heater can be cycled on and off to provide heat to the chuck and workpiece as required. In one embodiment, the chuck heaters include resistive elements driven by a DC power supply. In one particular embodiment, two or more individually controllable resistive heaters are used to facilitate an efficient multiple-stage, e.g., two-stage, chuck heating procedure, as described below.
In one embodiment, the efficiency of the system of the invention is improved by effectively dividing the operating temperature range of the system into multiple subranges. Operation of the system in a particular subrange defines a set of operating parameters which are used to control the system. For example, in one embodiment, the operating temperature range is divided into two subranges, an upper subrange and a lower subrange. Selection of a set point in one of the subranges defines a particular set of conditions. The system of the invention uses these conditions to define a set of system operating parameters and to control the various components of the system to realize these parameters. For example, in one embodiment, where operation in the upper subrange is desired, the chuck heater is used to heat the chuck, and all of the fluid flows adjacent to the base to maintain it at ambient temperature. To accomplish this operating configuration, the valves are operated to route the fluid to the base and the heater is cycled as required.
The temperature control system of the invention also includes a plurality of temperature sensors which provide temperature feedback at various locations throughout the system. In one embodiment, at least one temperature sensor is provided in the fluid path to monitor the temperature of the fluid. Temperature sensors can be provided in the fluid path to the upper portion of the chuck and in the path toward the base to provide independent temperature monitoring of both paths. A sensor can also be provided in the upper portion of the chuck to monitor the workpiece temperature. By monitoring both the fluid temperature and the workpiece temperature, a form of dual-loop control is provided for the workpiece temperature control. This allows for more precise temperature control. A sensor can also be provided in the base and in the ambient environment. These sensors along with the fluid sensor allow for dual-loop control of the base temperature to eliminate heat flow between the chuck and the host machine.
In another aspect, the invention is directed to the power and control system and method which provide the power and control required to operate the temperature control system of the invention. Specifically, this aspect of the invention is directed to a power and control system and method which provide the power to run system heaters and the control required to operate the various valves and fluid heaters in the temperature control system of the invention. The power system of the invention is applicable to a workpiece chuck which includes at least one heater for heating the workpiece mounted on the workpiece chuck. The power system includes an interface over which power is transferred between the power system and the workpiece chuck heater. The system also includes a switching power supply for generating an output power signal which is coupled to the chuck heater across the interface to power the chuck heater.
In one embodiment, the power system of the invention also includes a filter which receives the output power signal from the switching power supply. The filter filters the signal to remove switching power supply noise from the signal. As a result, the power signal supplied to the chuck heater does not couple noise to the workpiece and therefore allows for very accurate workpiece measurements.
In one embodiment, the output power signal from the switching power supply is controllable such that it can be switched between an ON state and an OFF state. In the ON state, the power signal is applied to the heater. In the OFF state, no power is applied to the heater such that the heater is turned off. The output power signal from the switching power supply can also be coupled to an amplifier which is controllable such that the output power signal level can be varied. As a result, power delivered to the heater, and, therefore, heat provided by the heater, can be varied.
As mentioned above, in one embodiment, the chuck can actually include two or more resistive heating elements used to supply heat to the workpiece, and the switching power supply can provide two or more output power signals, each of which is connected to its own respective heating element. The output power signals can be separately controllable such that power supplied to the heating elements can be separately controlled. In this embodiment, the heating elements and the output signals can be configured to efficiently control changing the temperature of the workpiece by implementing a multiple-stage, e.g., two-stage, heating procedure. For example, in a two-heating-element configuration, where it is desired to rapidly raise the temperature of the workpiece a substantial amount, both power outputs and, therefore, both heating elements can be activated simultaneously. As the temperature of the workpiece increases beyond a first target threshold temperature, one of the heating elements can be turned OFF while the other continues to be used to adjust the temperature of the workpiece to a desired final target temperature.
In one embodiment, to implement this two-stage procedure, the first output power signal is switchable between ON and OFF states, and the other is applied to a controllable amplifier such that its level can be varied. While increasing the temperature to the first target temperature, both heaters can be turned ON. As the target threshold temperature is reached, the first heater can be switched OFF while the second adjustable heater remains ON. The amplifier can then be controlled to adjust the level of the output power signal delivered to the adjustable heater in order to accurately and precisely heat the workpiece to and maintain its temperature at the final target temperature. In another embodiment, both heaters can be left ON, even when the workpiece has reached its final temperature, particularly when it is required to maintain the workpiece at a high temperature. In yet another embodiment, three heaters can be used to implement the two-stage heating process; two of the heaters can be switchable, and the third can be variable. At the final temperature, one of the switchable heaters can be switched OFF and the other switchable heater and the variable heater can be left ON to maintain the workpiece at the final temperature.
The power and control system of the invention can also provide driver signals to control the valves and fluid heaters used in the temperature control system of the invention to control the temperature of the chuck and workpiece. In one embodiment, the control signals are relay driver signals which drive relays which, in turn, control operation of the valves and fluid heaters. The control system can receive inputs from the temperature sensors, a dewpoint alarm sensor switch and a fluid level switch and can use these inputs to generate the control signals required to control the system components to operate the system as required.
The temperature control of the invention provides numerous advantages over prior art systems. For example, because the power system of the invention uses switching power supplies, the system is far more energy efficient than systems which use linear power supplies. Also, because the switching power supply is power-factor corrected, it complies with new European power standards. Because of the unique filtering of the invention, the power supply outputs are realized with minimal noise. The temperature control system itself also provides advantages over prior art temperature control systems. For example, because the temperature of the base between the upper portion of the chuck and the host machine is maintained at ambient temperature, heat flow between the chuck and the host machine is substantially eliminated. Also, the use of plural subranges dividing the overall temperature range of the system provides for improved system efficiency. By dividing the temperature range of the system into smaller subranges, different control settings carefully tailored to a smaller temperature subrange can be employed. Again, this results in greater system efficiency.